1. The Field Of the Invention
The present invention pertains to a method and apparatus to determine the flow regime within a pipe using passive acoustic techniques.
2. The Prior Art
The ideal multi-phase flow meter would determine composition of flowing fluids and the flow rates of each phase, without impeding the flow and/or reacting with the fluids contained within a pipe. This meter would be capable of use in extreme temperatures, pressures and hostile chemical environments while providing accurate results. The construction of this meter would be such that it would be simple and suitable for field applications as well as usage in the laboratory.
Historically, there are devices that perform some of these measurements and a few can operate in environments approaching extreme hostility. For example, nuclear densitometer techniques (see U.S. Pat. No. 4,683,759) are a reliable and robust means of obtaining an average fluid density in pipes containing flowing fluids. Since the instrument is externally mounted on the pipe, it would not interfere with or react with the flowing fluids. Instrument construction typically allows for usage in hostile conditions. The shortcomings of this approach to fluid characterizations are: the statistical nature of the measurements; the necessity of long lived and often high energy radioactive sources; and potential interpretational difficulties when the pipe under investigation contains gases, liquids and fluctuations in chemical composition.
An additional example of the prior art and applications to fluid characterization involves the use of ultrasonic techniques. These types of measurement systems can be intrusive or non-intrusive, depending on the application. Composition of two-phase fluids can be investigated using an intrusive transit time method, as described in U.S. Pat. No. 5,115,670. This method contemplates the measurement of the transit time of a sound wave between an ultrasonic source and a detector located diagonally across a pipe. In principle this transit time can be used to calculate the speed of sound in two-phase flow. This allows the calculation of the mixture's linear velocity and composition. These quantities allow the calculation of mass flow rates or the energy flow rates. The calculated results and their accuracy, for example steam quality, may depend on separate fluid property correlations. The fluid's chemical composition may effect the sensor's longevity.
Fluid velocities can be obtained using Doppler flow meters (see U.S. Pat. No. 5,115,670). These ultrasonic devices can be non-intrusive (externally mounted in the pipe) and protected from the environment. The idea behind these devices is that an ultrasonic signal is continuously transmitted into a pipe containing fluids where scattering occurs from suspended solids, air bubbles, discontinuities or disturbances in the flowing stream. The scattered signal is detected and its frequency is compared to the transmitted frequency. The difference in these frequencies is proportional to the fluid's velocity. These measurements are considered most accurate when evaluating fluids with Newtonian flow profiles and containing suspended particles or air bubbles.
Generally, the designs of existing flow measurement systems using nuclear, acoustic or electromagnetic methods only address a few of the idealized capabilities and concentrate on measuring a restricted set of parameters while actively probing the medium of interest. These measurement systems can be intrusive or non-intrusive and some may require a side stream sample to obtain the required data. Examples of some of the active acoustic flow measurement systems can be found in U.S. Pat. Nos. 4,080,837; 4,236,406; and 4,391,149.
Passive types of measurement techniques in pipes, specifically simple detection of acoustic emissions or "listening," are available, but are limited in scope and applications. For instance, acoustic emissions can be used to detect: slug flow and the presence of sand in multi-phase pipelines (see U.S. Pat. No. 5,148,405); leaks in natural gas pipelines (see U.S. Pat. No. 5,117,676); and steam quality when the acoustic emissions are obtained from a calibrated steam jet produced by an orifice (see U.S. Pat. No. 4,193,290). The use of acoustic emissions as a passive and non-intrusive method in quantitative characterization of multi-phase flow in pipes appears to be novel.